The present invention relates to a coupling for rigid attachment to metallic tubing. In particular the invention relates to a coupling having an interior configuration such that it can be swaged to such tubing in a pressure tight relationship while satisfying all mechanical and performance requirements for such swaged joints.
In many applications of hydraulic systems such as used in aircraft and aerospace applications, the end fittings for the tubing in such systems are flareless fittings with swaged on couplings or sleeves. Such sleeves have been characterized by internal annular grooves and were developed for use with light weight, thin wall, hydraulic tubing to eliminate the leakage and inadequate tensile joint strength problems encountered with bite type sleeves.
Among the couplings currently in use is a sleeve whose internal annular grooves are rectangular in shape, such as those described in U.S. Pat. No. 4,147,385, to Van der Velden, assignor to Boeing. The disclosures therein are incorporated by reference herein. In the sleeve described in U.S. Pat. No. 4,147,385, each groove has a base and two side walls, each side wall meeting the base at about 90.degree. to form corners; each corner having a small radius of curvature.
Another type of coupling currently in use has internal ramped grooves, such as those described in U.S. Pat. No. 4,026,006 to Mobius, which is incorporated by reference herein. In the sleeve described in U.S. Pat. No. 4,026,006, one wall of the groove meets the base at an angle greater than 90.degree., and a second wall meets the base at an angle of about 90.degree..
Currently used couplings with rectangular grooves are less than optimal for a number of reasons. Because of the small radius of curvature at the corners of the grooves where the sidewalls meet the base, tubing being swaged into the rectangular grooves tends to develop cracks. This is especially a problem with titanium tubing. Titanium does not like to flow into rectangular grooves. In fact, when swaged into rectangular grooves, the tubing deforms into a trapezoidal shape, leaving areas of highly stressed and unsupported material in shear. The swaged tubing therefore tends to develop cracks and leaks.
Theoretically, ramped grooves can be preferable to rectangular grooves for use in swaged contact with tubing. This is because the ramped groove allows the tubing material to flow smoothly into the grooves resulting in continuous grain flow. This continuous grain flow reduces shear stresses in the tube material. In addition, the ramp causes a more gradual transition from swaged to unswaged material than a rectangular groove, thus reducing the number of stress points and stress risers in the swaged tubing. The theoretical advantages of the ramped groove, such as those in the coupling sleeve of U.S. Pat. No. 4,026,006, are limited by the fact that currently used coupling sleeves are usually swaged only by force from outside the sleeve. External swaging fails to take advantage of the highly desirable compressive stresses which can be exerted on the outside of the tubing when swaging is internal. Therefore, currently used coupling sleeves with ramped grooves tend to induce an unacceptable amount of cracking in tubing to which they are swaged. Currently used couplings with ramped grooves also tend to vibrate loose in aerospace use, causing premature failures.
There exists therefore the need for improved couplings for swaged attachment to thin wall, flareless tubing which have improved metal fatigue characteristics, which do not tend to vibrate loose from the tubing to which they are swaged, and which are internally swaged to the tubing.